The National Institute for Occupational Safety and Health (NIOSH)NIOSH Mining

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The National Institute for Occupational Safety and Health (NIOSH) Office of Mine Safety and Health Research (OMSHR) has conducted research to evaluate the effectiveness of purging of mine refuge alternatives (RAs). Two questions were addressed experimentally: (1) Does the current generation of mobile refuge alternatives meet the requirements of 30 CFR § 7.508 (c) (2) which requires RAs to be capable of purging the internal atmosphere from 400 ppm of carbon monoxide (CO) to 25 ppm? (2) What is the relationship between the concentration of noxious gases in the mine atmosphere external to the refuge alternative and the concentration that will be present inside the refuge alternative following entry of miners but prior to purging? The goal of the second question was to evaluate the appropriateness of the 400-ppm criterion, given that ambient post-accident mine concentrations of CO can be in the thousands of ppm.

A tent-type and a rigid steel mobile refuge alternative were used to investigate the first question. Carbon monoxide (CO) and sulfur hexafluoride (SF6) were used as contaminant gases as part of this study, and the individual experiments were conducted with the purging area of the RA occupied by zero, one, or seven simulated (when CO was used) or live (when SF6 was used) occupants.

To investigate the second question, the aforementioned RAs were used along with a third airlock constructed for and employed in the experiments. The volume and size of the entry door into the constructed airlock were roughly in the middle of the range of values for the rigid and tent-type RAs. The RAs and constructed airlock were placed in a large sealed room, and SF6 gas was released into the room as a surrogate for CO. Experiments were conducted to determine the gas concentrations inside the airlock after groups of test subjects (representing miners) had entered. This demonstrates how much of the mine ambient CO ends up in the purging room following the entry of miners.

The experimental findings indicate that the current generation of mobile refuge alternatives are capable of purging the internal atmosphere from 400 ppm of CO to 25 ppm as required by 30 CFR § 7.508 (c) (2). The findings also demonstrate that the starting concentration of CO in the airlock of a mobile RA can be significantly greater than 400 ppm—e.g., in the thousands rather than the hundreds of ppm—and that a portion of the remaining CO after purging will be carried into the main chamber. Recommendations to mitigate this potential hazard are presented in the report.

This publication was revised on March 20, 2014, to correct an editorial omission in the recommendations on page 42. If you have a version of the document downloaded prior to that date, please download and use the corrected version.